Backscattering optical amplification device, optical pulse testing device, backscattering optical amplification method and optical pulse testing method

ABSTRACT

The present invention is to provide a backscattered light amplification device, an optical pulse test apparatus, a backscattered light amplification method, and an optical pulse test method for amplifying a desired propagation mode of Rayleigh backscattered light with a desired gain by stimulated Raman scattering in a fiber under test having the plurality of propagation modes. The backscattered light amplification device according to the present invention is configured to control individually power, incident timing, and pulse width of a pump pulse for each propagation mode when the pump pulse is incident in a plurality of propagation modes after the probe pulse is input to the fiber under test in any propagation mode.

TECHNICAL FIELD

The present disclosure relates to a backscattered light amplificationdevice and a backscattered light amplification method for amplifyingbackscattered light in an optical pulse test used to detectcharacteristics of an optical fiber, and relates to an optical pulsetest apparatus and an optical pulse test method using the same.

BACKGROUND ART

An optical pulse test method (herein, also referred to as an opticaltime domain reflectometer; OTDR) is well known as a test technique foran optical fiber. The OTDR indicates a method and a device that makespulsed test light incident on a fiber under test (hereinafter, referredto as FUT) and acquires distribution data (OTDR waveform) based onintensity and round trip time of backscattered light of Rayleighscattered light derived from a test light pulse propagating in anoptical fiber and Fresnel reflection light. Such a technique can be usedto detect an abnormal position at which breakage or increase in loss ofthe optical fiber occurs and to specify the position.

In Non-Patent Literature 1, a first higher-order mode (LP11 mode) ofbackscattered light is extracted using a wavelength region in which ageneral single-mode fiber (hereinafter, referred to as SMF) operates intwo modes, and a technique (1-μm-band mode-detection OTDR) is disclosedwhich detects an optical fiber bending with higher sensitivity than ageneral-purpose OTDR. Furthermore, Non-Patent Literature 2 discloses amethod of measuring both a fundamental mode (LP01 mode) and LP11 mode ofbackscattered light in a 1-μm-band mode-detection OTDR, evaluating theratio of loss occurring in the modes, and identifying factors of theloss occurring in the optical fiber.

CITATION LIST Non-Patent Literature

Non-Patent Literature 1: A. Nakamura, K. Okamoto, Y. Koshikiya, T.Manabe, M. Oguma, T. Hashimoto and M. Itoh, “High-sensitivity detectionof fiber bends: 1-μm-band mode-detection OTDR”, J. Lightw. Technol.,vol. 33, no. 23, pp. 4862-4869, 2015.

Non-Patent Literature 2: A. Nakamura, K. Okamoto, Y. Koshikiya, T.Manabe, M. Oguma, T. Hashimoto, and M. Itho, “Loss Cause Identificationby Evaluating Backscattered Modal Loss Ratio Obtained With 1-μm-BandMode-Detection OTDR”, J. Lightw. Technol., vol. 34, no. 15, pp.3568-3576, 2016.

Non-Patent Literature 3: D. M. Spirit and L. C. Blank, “Raman-assistedlong-distance optical time domain reflectometry,” Electron. Lett., vol.25, pp. 1687-1689, Dec. 1989.

Non-Patent Literature 4: Christensen E N, Koefoed J G, Friis S M M,Castaneda M A U, Rottwitt K. “Experimental characterization of Ramanoverlaps between mode-groups,” Scientific Reports. 2016; 6:34693.doi:10.1038/srep34693.

SUMMARY OF THE INVENTION Technical Problem

Non-Patent Literature 3 proposes a method, as a method of expanding ameasurement distance in OTDR measurement, of amplifying distributedly inFUT backscattered light generated by a probe pulse propagating in FUTusing pump light having a frequency (short wavelength) higher by theRaman frequency shift.

However, when the FUT is a two-mode region of Few-mode optical fiberhaving a plurality of propagation modes and a general SMF, it is known(Non-Patent Literature 4) that an amplification gain due to stimulatedRaman scattering may differ depending on two propagation modes of lightinteracting with each other. For this reason, there is a problem in OTDRmeasurement in the two-mode region of the Few-mode optical fiber and thegeneral SMF that a method of amplifying a desired propagation mode ofthe backscattered light with a desired gain through stimulated Ramanscattering is unclear.

Therefore, in order to solve the above-described problem of the relatedart, the present invention is to provide a backscattered lightamplification device, an optical pulse test apparatus, a backscatteredlight amplification method, and an optical pulse test method in which adesired propagation mode of Rayleigh backscattered light is amplifiedwith a desired gain by stimulated Raman scattering in the fiber undertest having a plurality of propagation modes.

Means for Solving the Problem

In order to achieve the above object, a backscattered lightamplification device according to the present invention is configured tocontrol individually power, incident timing, and pulse width of the pumppulse for each propagation mode when the pump pulse is incident in aplurality of propagation modes after the probe pulse is input to a fiberunder test in any propagation mode.

Specifically, a backscattered light amplification device according tothe present invention includes:

-   -   a probe pulse incidence means for making a probe pulse incident        on one end of a fiber under test in a desired propagation mode;    -   a pump pulse incidence means for making a pump pulse, which        generates a Raman gain spectrum in an optical frequency range        including an optical frequency of the probe pulse, incident on        the one end of the fiber under test in a plurality of        propagation modes after the probe pulse incidence means makes        the probe pulse incident on the fiber under test; and    -   a control means for setting a power ratio of the pump pulse        between the propagation modes, a length of the pump pulse in        each of the propagation modes, and a relative time difference        between the probe pulse incident on the fiber under test and the        pump pulse in each of the propagation modes so as to give a        desired Raman amplification gain to backscattered light of a        desired propagation mode generated at a spot far from a desired        spot of the fiber under test, among backscattered light of a        plurality of propagation modes generated from the probe pulse        propagating in the fiber under test.

A backscattered light amplification method according to the presentinvention includes:

-   -   a probe pulse incidence procedure for making a probe pulse        incident on one end of a fiber under test in a desired        propagation mode;    -   a pump pulse incidence procedure for making a pump pulse, which        generates a Raman gain spectrum in an optical frequency range        including an optical frequency of the probe pulse, incident on        the one end of the fiber under test in a plurality of        propagation modes after the probe pulse is incident on the fiber        under test in the probe pulse incidence procedure; and    -   a control procedure for setting a power ratio of the pump pulse        between the propagation modes, a length of the pump pulse in        each of the propagation modes, and a relative time difference        between the probe pulse incident on the fiber under test and the        pump pulse in each of the propagation modes so as to give a        desired Raman amplification gain to backscattered light of a        desired propagation mode generated at a spot far from a desired        spot of the fiber under test in the pump pulse incidence        procedure, among backscattered light of a plurality of        propagation modes generated from the probe pulse propagating in        the fiber under test.

Since the power, incident timing, and pulse width of the pump pulse areindividually controlled for each propagation mode, it is possible togive an arbitrary Raman amplification gain from any spot (spot in alongitudinal direction of the fiber under test) to a desired propagationmode of the backscattered light generated by the probe pulse.

Accordingly, the present invention can provide the backscattered lightamplification device and the backscattered light amplification methodfor amplifying the desired propagation mode of the Rayleighbackscattered light with a desired gain by stimulated Raman scatteringin the fiber under test having the plurality of propagation modes.

The backscattered light amplification device according to the presentinvention further includes a mode demultiplexing means for separatingthe backscattered light returning to the one end of the fiber under testfor each propagation mode.

The backscattered light amplification method according to the presentinvention further includes a mode demultiplexing procedure forseparating the backscattered light returning to the one end of the fiberunder test for each propagation mode.

An optical pulse test apparatus according to the present inventionincludes:

-   -   the backscattered light amplification device; and    -   an arithmetic processing device that acquires, for each        propagation mode, a light intensity distribution in a length        direction of the fiber under test from the backscattered light        returning to the one end of the fiber under test, wherein    -   the arithmetic processing device is configured to    -   operate the backscattered light amplification device to acquire        a first light intensity distribution when the probe pulse and        the pump pulse are incident on the fiber under test,    -   operate the pump pulse incidence means and the mode        demultiplexing means of the backscattered light amplification        device to acquire a second light intensity distribution when        only the pump pulse is incident on the fiber under test, and    -   subtract, for each propagation mode, the second light intensity        distribution from the first light intensity distribution to        acquire a third light intensity distribution that would occur        when only the probe pulse is incident on the fiber under test.

An optical pulse test method according to the present invention ofperforming:

-   -   the backscattered light amplification method;    -   an arithmetic processing method of acquiring, for each        propagation mode, a light intensity distribution in a length        direction of the fiber under test from the backscattered light        returning to the one end of the fiber under test, the optical        pulse test method comprising of:    -   using the backscattered light amplification method and the        arithmetic processing method to acquire a first light intensity        distribution when the probe pulse and the pump pulse are        incident on the fiber under test;    -   using the pump pulse incidence procedure and the mode        demultiplexing procedure of the backscattered light        amplification method and the arithmetic processing method to        acquire a second light intensity distribution when only the pump        pulse is incident on the fiber under test; and    -   subtracting, for each propagation mode, the second light        intensity distribution from the first light intensity        distribution to acquire a third light intensity distribution        that would occur when only the probe pulse is incident on the        fiber under test.

In the backscattered light amplification device and method, it ispossible to exclude the backscattered light component of the pump pulse,which is a noise component, and to observe the backscattered light inthe desired propagation mode.

The inventions described above can be combined as much as possible.

Effects of the Invention

The present invention can provide a backscattered light amplificationdevice, an optical pulse test apparatus, a backscattered lightamplification method, and an optical pulse test method in which adesired propagation mode of Rayleigh backscattered light is amplifiedwith a desired gain by stimulated Raman scattering in a fiber under testhaving a plurality of propagation modes.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of an optical pulsetest apparatus according to the present invention.

FIG. 2 is a diagram illustrating a mode multiplexer/demultiplexerincluded in the optical pulse test apparatus according to the presentinvention.

FIG. 3 is a diagram illustrating stimulated Raman amplification of abackscattered light amplification device according to the presentinvention.

FIG. 4 is a diagram illustrating stimulated Raman amplification of abackscattered light amplification device according to the presentinvention.

FIG. 5 is a diagram illustrating an optical frequency arrangement oflight by the backscattered light amplification device according to thepresent invention.

FIG. 6 is a flowchart illustrating an optical pulse test methodaccording to the present invention.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention will be described with referenceto the accompanying drawings. The embodiment described below is anexample of the present invention, and the present invention is notlimited to the following embodiment. In the description and thedrawings, components having the same reference numerals indicate thesame components.

FIG. 1 is a diagram illustrating an optical pulse test apparatus of apresent embodiment. In the embodiment, it is assumed that a fiber undertest propagates only two propagation modes of a fundamental mode and afirst higher-order mode and a propagating light is in a singlepolarization state. The optical pulse test apparatus includes abackscattered light amplification device 10 and an arithmetic processingdevice 20 which acquires a light intensity distribution in a lengthdirection of a fiber under test FUT in every propagation mode, from thebackscattered light that returns to one end of the fiber under test FUT

The backscattered light amplification device 10 includes;

-   -   a probe pulse incidence means 11 for making a probe pulse P0        incident on one end of a first under test FUT in a desired        propagation mode;    -   a pump pulse incidence means 12 for making pump pulses P1 and        P2, which generate a Raman gain spectrum in an optical frequency        range including an optical frequency of the probe pulse P0,        incident on the one end of the fiber under test FUT in a        plurality of propagation modes after the probe pulse incidence        means 11 makes the probe pulse P0 incident on the fiber under        test FUT; and    -   a control means 13 for setting a power ratio of the pump pulse        between the propagation modes, a length of the pump pulse in        each of the propagation modes, and relative time differences ΔT1        and ΔT2 between the probe pulse P0 incident on the fiber under        test FUT and the pump pulse in each of the propagation modes so        as to give a desired Raman amplification gain to backscattered        light of a desired propagation mode generated at a spot far from        a desired spot of the fiber under test FUT, among backscattered        light of a plurality of propagation modes generated from the        probe pulse P0 propagating in the fiber under test FUT.

The backscattered light amplification device 10 further includes a modedemultiplexing means (mode multiplexer/demultiplexer 1-13) forseparating the backscattered light returning to the one end of the fiberunder test FUT for each propagation mode.

In FIG. 1, reference numeral 1-1 indicates a first light source thatemits light of optical frequency ν1, reference numeral 1-2 indicates afirst optical pulser configured to pulse the light emitted from thefirst light source to generate a probe pulse, reference numeral 1-3indicates a second light source that emits light of optical frequencyν2, reference numeral 1-4 indicates a variable optical splitter thatsplits the light emitted from the second light source at a desiredratio, reference numerals 1-5 and 1-6 indicate a second and thirdoptical pulsers configured to pulse the light split by the variableoptical splitter to generate first and second pump pulses, referencenumerals 1-7, 1-8 and 1-9 indicate electric pulse generators thatgenerate an electric pulse for modulating the light input to each of thefirst, second and third optical pulsers, reference numeral 1-10indicates an optical multiplexer that multiplexes the probe pulse andthe first pump pulse, reference numeral 1-11 indicates a first opticalcirculator that divide a fundamental mode component of the backscatteredlight, reference numeral 1-12 indicates a second optical circulator thatdivide a first higher-order mode component of the backscattered light,reference numeral 1-13 indicates a mode multiplexer/demultiplexerconfigured to make the multiplexed probe pulse and the first pump pulse,and the second pump pulse incident on the fiber under test in thefundamental mode and the first higher-order mode, respectively, and toseparate the backscattered light from the fiber under test into thefundamental mode component and the first higher-order mode component,reference numerals 1-14 and 1-15 indicate optical bandpass filters thatremove the backscattered light generated by the first and second pumppulses from the backscattered light, reference numerals 1-16 and 1-17indicate photodetectors, reference numerals 1-18 and 1-19 indicate A/Dconverters, and reference numeral 1-20 indicates an arithmeticprocessor.

A probe pulse incidence means 11 includes the first light source 1-1,the first optical pulser 1-2, the electric pulse generator 1-7, theoptical multiplexer 1-10, the first optical circulator 1-11, and themode multiplexer/demultiplexer 1-13.

A pump pulse incidence means 12 includes the second light source 1-3,the variable optical splitter 1-4, the second optical pulser 1-5, thethird optical pulser 1-6, the electric pulse generator 1-8, the electricpulse generator 1-9, the optical multiplexer 1-10, the first opticalcirculator 1-11, the second optical circulator 1-12, and the modemultiplexer/demultiplexer 1-13.

A control means 13 controls the variable optical splitter 1-4, theelectric pulse generator 1-7, the electric pulse generator 1-8, and theelectric pulse generator 1-9.

An arithmetic processing device 20 includes the optical bandpass filters1-14 and 1-15, the photodetectors 1-16 and 1-17, the A/D converters 1-18and 1-19, and the arithmetic processor 1-20.

Further, since the gain bandwidth due to stimulated Raman scattering isrelatively wide, the optical pulse test apparatus according to thepresent invention can use a general-purpose DFB laser or the like for afirst light source 1-1 that is a probe pulse light source, so that theRayleigh backscattered light of the probe pulse receives the sufficientamplification gain even when the spectral linewidth is wide. In thiscase, since heterodyne detection cannot be used, direct detection willbe used.

A description will be theoretically given with respect to the fact thatthe backscattered light amplification device 10 can amplify any desiredpropagation mode of backscattered light generated by a probe pulse bycontrolling the gain of the stimulated Raman scattering generated by thefirst and second pump pulses.

The light having the optical frequency ν1 emitted from the first lightsource 1-1 is pulsed by the optical pulser 1-2 based on the electricsignal generated by the electric pulse generator 1-7 to generate a probepulse. On the other hand, the light having the optical frequency ν2emitted from the second light source 1-3 is split by the variableoptical splitter 1-4 at a desired ratio, and is pulsed by the opticalpulsers 1-5 and 1-6 based on the electric signals generated by theelectric pulse generators 1-8 and 1-9, respectively to generate firstand second pump pulses. A frequency difference (Δν=ν2−ν1) between theoptical frequency ν1 of the probe pulse and the optical frequency ν2 ofthe first and second pump pulses is set to match the Raman gain band ofthe fiber under test.

The first pump pulse is multiplexed with the probe pulse by the opticalmultiplexer 1-10, and is input to the mode multiplexer/demultiplexer1-13 by passing through the first optical circulator 1-11. The secondpump pulse is input to the mode multiplexer/demultiplexer 1-13 bypassing through the second optical circulator 1-12.

The mode multiplexer/demultiplexer 1-13 outputs an LP01 mode, which isinput to port 1, to port 3 without change of the LP01 mode as shown inFIG. 2, converts an LP01 mode input to port 2 into an LP11 mode, andoutputs the LP11 mode from port 3. In addition, the modemultiplexer/demultiplexer 1-13 outputs, for the LP01 mode and the LP11mode, which are input to port 3, the LP01 mode to port 1 without changeof the LP01 mode, converts the LP11 mode into the LP01 mode, and outputsthe LP01 mode to port 2. When such a mode multiplexer/demultiplexer 1-13is used in the backscattered light amplification device 10, as describedabove, the multiplexed probe pulse and first pump pulse can be incidenton the fiber under test FUT in the LP01 mode, and the multiplexed secondpump pulse can be incident on the fiber under test FUT in the LP11 modein such a manner that the first optical circulator 1-11 is connected toport 1 and the second optical circulator 1-12 is connected to port 2.Further, when the second optical circulator 1-12 is connected to port 1and the first optical circulator 1-11 is connected to port 2, contraryto the above description, the multiplexed probe pulse and first pumppulse can be incident on the fiber under test FUT in the LP11 mode, andthe multiplexed second pump pulse can be incident on the fiber undertest FUT in the LP01 mode.

The multiplexed probe pulse, first pump pulse, and second pump pulse areinput from one end of the fiber under test FUT. The probe pulse isincident earlier in time, the first pump pulse is incident in successionwith a time delay ΔT1, and the second pump pulse is incident with a timedelay ΔT2. A pulse width of the probe pulse, pulse widths of the firstand second pump pulses, and the relative time delays ΔT1 and ΔT2 betweenthe probe pulse and the first and second pump pulses can be adjusted bythe electric pulse generators 1-7, 1-8, and 1-9.

As shown in FIG. 3, when the probe pulse and the first and second pumppulses are incident on the fiber under test FUT, backscattered light isgenerated by the probe pulse incident precedingly. For example, when thewavelength of the probe pulse is in a two-mode region (the fundamentalmode and the first higher-order mode can propagate) below the cutoffwavelength of a general single-mode optical fiber, the backscatteredlight is coupled into the fundamental mode and the first higher-ordermode, and propagate toward the input end of the fiber under test FUT.Since Rayleigh scattering is elastic scattering, the optical frequencydoes not change due to the scattering process, and the generatedbackscattered light and probe pulse have the same optical frequency ν1.

Subsequently, the first and second pump pulses propagate through thefiber under test FUT while following the probe pulse. When thebackscattered light generated by the probe pulse encounters the pumppulse and the backscattered light exists within the Raman amplificationgain band generated by the pump pulse, stimulated Raman amplification ofthe backscattered light due to the pump pulse occurs, and thebackscattered light is amplified.

As shown in FIG. 4, when the fiber under test FUT is an optical fiber inwhich a plurality of propagation modes exist or each of the pulses is ina wavelength region where the plurality of propagation modes propagate,the fundamental mode of the backscattered light is subjected to apredetermined Raman amplification gain from both the pump pulse (firstpump pulse) input in the LP01 mode and the pump pulse (second pumppulse) input in the LP11 mode. In addition, the LP11 mode of thebackscattered light is also subjected to a predetermined Ramanamplification gain from both the pump pulse (first pump pulse) input inthe LP01 mode and the pump pulse (second pump pulse) input in the LP11mode.

The backscattered light of the fundamental mode and the firsthigher-order mode returned to the incident end of the fiber under testFUT is separated by the mode multiplexer/demultiplexer 1-13, and thenextracted through the circulators 1-11 and 1-12. The backscattered lightgenerated by the first and second pump pulses out of the extractedbackscattered light is removed by the optical band pulse filters 1-14and 1-15. The extracted backscattered light of the fundamental mode andthe first higher-order mode is converted into an electric signal by thephotodetectors 1-16 and 1-17, digitized by the A/D converters 1-18 and1-19, and analyzed by the arithmetic processor 1-20.

When the input end (one end) of the fiber under test FUT is defined asz=0, powers P₀₁ (z) and P₁₁ (z) of the LP01 mode and the LP11 mode ofthe backscattered light generated by the probe pulse at a spot z areexpressed as follows

[Formula 1]

P ₀₁(Z)=R ₀₁−₀₁ P ₀(0)exp(−α₀₁ z) . . .  (1)

and

[Formula 2]

P ₁₁(Z)=R ₀₁−₁₁ P ₀(0)exp(−α₀₁ z) . . .  (2)

Here, the symbol P₀ (0) indicates an optical power at the incident endof the probe pulse, the symbol an indicates a loss factor of the LP01mode of the fiber under test FUT, and the symbols R₀₁₋₀₁ and R₀₁₋₁₁indicate coupling ratios of the LP01 mode of the probe pulse withrespect to the LP01 mode and the LP11 mode of the backscattered light.

The LP01 mode of the backscattered light returning to the input end issubjected to the following Raman amplification G01 from the first (LP01mode) and second (LP11 mode) pump pulses.

[Formula 3]

G ₀₁=exp{γ₀₁₋₀₁ ηP _(pump) ⁰¹(z−Δz ₁)ΔL ₁+γ₁₁₋₀₁(1−η)P _(pump) ¹¹(z−Δz₂)ΔL ₂} . . .  (3)

Here, the symbols γ01-01 and γ11-01 indicate mode-dependent gainefficiencies in which the LP01 mode of the backscattered light issubjected to a gain from the first (LP01 mode) and second (LP11 mode)pump pulses by stimulated Raman scattering. The symbol η indicates asplitting ratio of the variable optical splitter 1-4, that is, a ratiobetween the powers of the first and second pump pulses.

[Formula 3-1]

P _(pump) ⁰¹(z−Δz ₁) . . .  (3-1)

The symbol expressed by Formula 3-1 indicates an optical power of thefirst (LP01 mode) pump pulse at a spot z-Δz1.

[Formula 3-2]

P _(pump) ¹¹(z−Δz ₂) . . .  (3-2)

The symbol expressed by Formula 3-2 indicates an optical power of thesecond (LP11 mode) pump pulse at a spot z-Δz2.

The symbols ΔL1 and ΔL2 indicate interaction lengths at which thebackscattered light of the probe pulse subjected to the stimulated Ramanamplification by the first and second pump pulses, and can be adjustedby the pulse width of the pump pulse.

The LP11 mode of the backscattered light is subjected to the followingRaman amplification G11 from the first (LP01 mode) and second (LP11mode) pump pulses.

[Formula 4]

G ₁₁=exp{γ₀₁₋₁₁ ηP _(pump) ⁰¹(z−Δz ₁)ΔL ₁+γ₁₁₋₁₁(1−η)P _(pump) ¹¹(z−Δz₂)ΔL ₂} . . .  (4)

Here, the symbols γ01-11 and γ11-11 indicate mode-dependent gainefficiencies in which the LP11 mode of the backscattered light issubjected to a gain from the first (LP01 mode) and second (LP11 mode)pump pulses by stimulated Raman scattering.

The mode-dependent gain efficiency, that is, Raman amplificationefficiency between different modes is an overlap integral of across-sectional intensity distribution I of modes n and m, and isrepresented by the following formula.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack & \; \\{\gamma_{n,m} = \frac{\int{\int{{I_{n}\left( {x,y} \right)}{I_{m}\left( {x,y} \right)}{dxdy}}}}{\int{\int{{I_{n}\left( {x,y} \right)}{dxdy}{\int{\int{{I_{m}\left( {x,y} \right)}{dxdy}}}}}}}} & (5)\end{matrix}$

This is uniquely determined by a refractive index profile of the fiberunder test FUT.

Distances (½ of the pulse interval) Δz1 and Δz2 from the position wherethe backscattered light of the probe pulse is generated to positionswhere the backscattered light encounters the first and second pumppulses are expressed by the following formulas using the time delays ΔT1and ΔT2.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack & \; \\{{\Delta\; z_{1}} = \frac{{c \cdot \Delta}\; T_{1}}{2}} & (6) \\{{and}\left\lbrack {{Formula}\mspace{14mu} 7} \right\rbrack} & \; \\{{\Delta\; z_{2}} = \frac{{c \cdot \Delta}\; T_{2}}{2}} & (7)\end{matrix}$

Here, the symbol c indicates a speed of light in the fiber under testFUT.

By the control of time intervals ΔT1 and ΔT2 between the probe pulse andthe first and second pump pulses, the probe light can be amplified fromany spot. The speed of light in the fiber under test FUT slightlychanges depending on the propagation mode. However, normally, the pulsewidth (interaction length with the backscattered light) of the pumppulse is 10 km or more, whereas the change in Δz1 and Δz2 caused by thedifference in the speed of light of each mode is less than 10 m, andthus the difference in the speed of light for each mode can be ignored.

A power P₀₁ (t) of the LP01 mode of the backscattered light generated bythe probe pulse observed at a reception unit (z=0) of this device attime t is expressed as follows.

[Formula 8]

P ₀₁(t)=R ₀₁₋₀₁ P ₀(0)exp{−2α₀₁ z+γ ₀₁₋₀₁(z−Δz ₁)ΔL ₁+γ₁₁₋₀₁(1−η)P_(pump) ¹¹(z−Δz ₂)ΔL ₂} . . .  (8)

On the other hand, a power P₁₁ (t) of the LP11 mode of the backscatteredlight is expressed as follows.

[Formula 9]

P ₁₁(t)=R ₀₁₋₁₁ P ₀(0)exp{−(α₀₁+α₁₁)z+γ ₀₁₋₁₁ ηP _(pump) ⁰¹(z−Δz ₁)ΔL₁+γ₁₁₋₁₁(1−η)P _(pump) ¹¹(z−Δz ₂)ΔL ₂} . . .  (9)

Here, the symbol α₁₁ indicates a loss factor of the LP11 mode of thefiber under test FUT. Further, the following formula is satisfied.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 10} \right\rbrack & \; \\{z = \frac{c \cdot t}{2}} & (10)\end{matrix}$

As indicated in Formulas (8) and (9), by the control of the power ratioη between the first and second pump pulses, the interaction lengths ΔL1and ΔL2 between the probe pulse and the first and second pump pulses,and the relative time differences ΔT1 and ΔT2 between the probe pulseand the first and second pump pulses, any Raman amplification gain canbe applied from any spot to any mode of the backscattered light of theprobe pulse.

FIG. 5 schematically shows a frequency arrangement relation of the probepulse, the pump pulse, the backscattered light generated by each of thepulse, and the Raman gain spectrum generated by the pump pulse.

The Rayleigh backscattered light generated by the probe pulse has thesame optical frequency ν1 as the probe pulse. For the pump pulse havingthe optical frequency ν2, the Raman gain spectrum is around ν2-νrdownshifted by a Raman frequency shift νr of the fiber under test FUT.When the frequency ν1 of the probe pulse is within the Raman gainspectrum, the backscattered light of the probe pulse is subjected toRaman amplification in the fiber under test FUT.

FIG. 6 is a flowchart illustrating a waveform analysis procedure in thearithmetic processor 1-20.

First, in step S01, an LP01 mode waveform b1 and an LP11 mode waveformb2 of the backscattered light are acquired in a state where the probepulse and the first and second pump pulses are input. The waveforms b1and b2 observed at this time are defined as F_(G) ⁰¹ (z) and F_(G) ¹¹(z), respectively.

Next, in step S02, an LP01 mode waveform b3 and an LP11 mode waveform b4of the backward Raman scattered light having no stimulated componentgenerated only by the pump pulse are acquired in a state where only thefirst and second pump pulses are input. The waveforms observed at thistime are defined as F_(P) ⁰¹ (z) and F_(P) ¹¹ (z), respectively.

Finally, in step S03, the differences b1-b3 and b2-b4 of these waveformsare taken, and thus it is possible to acquire and analyze an LP01 modewaveform F_(R) ⁰¹ (z) and an LP11 mode waveform F_(R) ¹¹ (z) of theamplified Rayleigh backscattered light of the probe pulse.

The waveform F_(R) ⁰¹ (z) and the waveform F_(R) ¹¹ (z) are obtainedusing Formulas (8) and (9) as a function of the length direction z ofthe fiber under test FUT.

[Note]

The optical pulse test device according to the present invention will bedescribed below.

(Problem)

The present invention is to enable the backscattered light of aplurality of propagation modes propagating in the optical fiber to beindividually amplified by the stimulated Raman scattering.

(Means)

In order to solve the above problem, the optical pulse test apparatusincludes:

-   -   a first light source configured to output probe light having a        wavelength that can propagate in a fundamental mode and a first        higher-order mode of a fiber under test FUT;    -   a pulser configured to pulse the probe light to generate a probe        pulse;    -   a second light source configured to output pump light having a        wavelength shifted by a Raman frequency shift to a shorter        wavelength than the probe light;    -   a variable optical splitter configured to split the pump light        at a desired splitting ratio;    -   second and third pulsers configured to pulse the pump light        split by the variable optical splitter to generate first and        second pump pulses;    -   a signal generator configured to generate an electric pulse        train used to control the first, second, and third pulsers;    -   an optical multiplexer configured to multiplex the probe pulse        and a first pump pulse;    -   a first optical circulator configured to separate a fundamental        mode component of backscattered light from the fiber under test        FUT;    -   a second optical circulator configured to separate a first        higher-order mode of the backscattered light from the fiber        under test FUT;    -   a mode multiplexer/demultiplexer configured to make the        multiplexed probe pulse and the first pump pulse incident on the        fiber under test FUT in one of the fundamental mode and the        first higher-order mode and, the second pump pulse in the other        of the fundamental mode and the first higher-order mode, and to        separate the backscattered light from the fiber under test FUT        into the fundamental mode and the first higher-order mode;    -   a first optical bandpass filter configured to remove the        backscattered light of the pump pulse from the fundamental mode        of the backscattered light separated by the first optical        circulator;    -   a second optical bandpass filter configured to remove the        backscattered light of the pump pulse from the first        higher-order mode of the backscattered light separated by the        second optical circulator;    -   first and second photodetectors configured to photoelectrically        convert the backscattered light that has passed through the        first and second optical bandpass filters, respectively;    -   first and second A/D converters configured to convert        photocurrents output from the first and second photodetectors        into voltages, respectively; and an arithmetic processing unit        configured to acquire an intensity distribution of the        fundamental mode component of the backscattered light with        respect to a distance of the fiber under test FUT and an        intensity distribution of the first higher-order mode component        of return light with respect to a distance of the fiber under        assumption.

The pump light has a wavelength shifted by a Raman frequency shift to ashorter wavelength than the probe pulse. The optical pulse testapparatus controls the pump pulses excited in a mode-selective manner toan arbitrary optical power ratio, a pulse width, and a relative timedelay with respect to the probe pulse using the variable opticalsplitter, the second and third pulsers, and the modemultiplexer/demultiplexer, and amplifies the fundamental mode and thefirst higher-order mode of Rayleigh backscattered light of a probe pulsepreviously incident on the fiber under test FUT from a desired spot witha desired gain.

The arithmetic processor is configured to acquire a backscattered lightwaveform in first measurement in a state where the probe pulse and thefirst and second pump pulses are incident on the fiber under test FUT.

The arithmetic processor is configured to acquire backscattered lightwaveforms of the first and second pump pulses in second measurement in astate where the probe pulse is not incident on the fiber under test FUT.

The arithmetic processor is configured to calculate a Rayleighbackscattered light waveform of the probe pulse from a differencebetween the waveform acquired in the first measurement and the waveformsacquired in the second measurement.

Effects of the Invention

According to the optical pulse test apparatus and the optical pulse testmethod of the present invention, the conditions (input mode, inputpower, input timing, and pulse width) of the pump light to be input arecontrolled in the optical fiber where a plurality of propagation modesexist or in the optical fiber where each of the pulses is in awavelength region in which the plurality of propagation modes propagate,and thus the stimulated Raman scattering can be used to amplify adesired propagation mode of the Rayleigh backscattered light with adesired gain from a desired spot.

In the present embodiment described above, the fiber under testpropagates only two propagation modes of the fundamental mode and thefirst higher-order mode, and the propagating light is in a singlepolarization state. However, even when the fiber under test canpropagate three or more propagation modes and the propagating light isin a single polarization state, similarly, the desired propagation modeof the Rayleigh backscattered light can be amplified with a desired gainfrom a desired spot by the control of the conditions of the pump lightto be input.

REFERENCE SIGNS LIST

-   1-1 First light source-   1-2 First pulser-   1-3 Second light source-   1-4 Variable optical splitter-   1-5, 1-6 Second and third pulser-   1-7 to 1-9 Electric pulse generator-   1-10 Optical multiplexer-   1-11, 1-12 Optical circulator-   1-13 Mode multiplexer/demultiplexer-   1-14, 1-15 Optical bandpass filter-   1-16, 1-17 Photodetector-   1-18, 1-19 A/D converter-   1-20 Arithmetic processor-   10 Backscattered light amplification device-   11 Probe pulse incidence means-   12 Pump pulse incidence means-   13 Control means-   20 Arithmetic processing device

1. A backscattered light amplification device comprising: a probe pulseincidence means for making a probe pulse incident on one end of a fiberunder test in a desired propagation mode; a pump pulse incidence meansfor making a pump pulse, which generates a Raman gain spectrum in anoptical frequency range including an optical frequency of the probepulse, incident on the one end of the fiber under test in a plurality ofpropagation modes after the probe pulse incidence means makes the probepulse incident on the fiber under test; and a control means for settinga power ratio of the pump pulse between the propagation modes, a lengthof the pump pulse in each of the modes, and a relative time differencebetween the probe pulse incident on the fiber under test and the pumppulse in each of the propagation modes so as to give a desired Ramanamplification gain to backscattered light of a desired propagation modegenerated at a spot far from a desired spot of the fiber under test,among backscattered light of a plurality of propagation modes generatedfrom the probe pulse propagating in the fiber under test.
 2. Thebackscattered light amplification device according to claim 1, furthercomprising: a mode demultiplexing means for separating the backscatteredlight returning to the one end of the fiber under test for eachpropagation mode.
 3. An optical pulse test apparatus comprising: thebackscattered light amplification device according to claim 2; and anarithmetic processing device that acquires, for each propagation mode, alight intensity distribution in a length direction of the fiber undertest from the backscattered light returning to the one end of the fiberunder test, wherein the arithmetic processing device is configured tooperate the backscattered light amplification device to acquire a firstlight intensity distribution when the probe pulse and the pump pulse areincident on the fiber under test, operate the pump pulse incidence meansand the mode demultiplexing means of the backscattered lightamplification device to acquire a second light intensity distributionwhen only the pump pulse is incident on the fiber under test, andsubtract, for each propagation mode, the second light intensitydistribution from the first light intensity distribution to acquire athird light intensity distribution that would occur when only the probepulse is incident on the fiber under test.
 4. A backscattered lightamplification method comprising: a probe pulse incidence procedure formaking a probe pulse incident on one end of a fiber under test in adesired propagation mode; a pump pulse incidence procedure for making apump pulse, which generates a Raman gain spectrum in an opticalfrequency range including an optical frequency of the probe pulse,incident on the one end of the fiber under test in a plurality ofpropagation modes after the probe pulse is incident on the fiber undertest in the probe pulse incidence procedure; and a control procedure forsetting a power ratio of the pump pulse between the propagation modes, alength of the pump pulse in each of the propagation modes, and arelative time difference between the probe pulse incident on the fiberunder test and the pump pulse in each of the propagation modes so as togive a desired Raman amplification gain to backscattered light of adesired propagation mode generated at a spot far from a desired spot ofthe fiber under test in the pump pulse incidence procedure, amongbackscattered light of a plurality of propagation modes generated fromthe probe pulse propagating in the fiber under test.
 5. Thebackscattered light amplification method according to claim 4, furthercomprising: a mode demultiplexing procedure for separating thebackscattered light returning to the one end of the fiber under test foreach propagation mode.
 6. An optical pulse test method of performing:the backscattered light amplification method according to claim 5; andan arithmetic processing method of acquiring, for each propagation mode,a light intensity distribution in a length direction of the fiber undertest from the backscattered light returning to the one end of the fiberunder test, the optical pulse test method comprising of: using thebackscattered light amplification method and the arithmetic processingmethod to acquire a first light intensity distribution when the probepulse and the pump pulse are incident on the fiber under test; using thepump pulse incidence procedure and the mode demultiplexing procedure ofthe backscattered light amplification method and the arithmeticprocessing method to acquire a second light intensity distribution whenonly the pump pulse is incident on the fiber under test; andsubtracting, for each propagation mode, the second light intensitydistribution from the first light intensity distribution to acquire athird light intensity distribution that would occur when only the probepulse is incident on the fiber under test.